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An Icy Riddle as Big as Greenland

This vaulting heap of ice and the swirling seas nearby have emerged as vital pieces of an urgent puzzle posed by global warming. Can the continuing slow increase in worldwide temperatures touch off abrupt climate upheavals?

Each piece of the puzzle is a dynamic and complicated body of water. One, the North Atlantic, is some two miles deep and liquid. The other, this ice cap, is two miles high and solid. For scale, think of it as a freshwater Gulf of Mexico that has been frozen, inverted and plunked atop the world's largest island.

Experts have reported a series of observations in recent months that show that the ice and the waters here are in a state of profound flux. If the trends persist, they could mean higher sea levels and widespread coastal flooding. There is also a small chance that the changes could lead to a sharp cooling in parts of the Northern Hemisphere.

Although nobody expects shifts as rapid or cataclysmic as portrayed in the new movie ''The Day After Tomorrow,'' the cooling could disrupt the relatively stable climatic conditions in which modern human societies have evolved.

In the last few years, Greenland's melt zone, where summer warmth turns snow on the edge of the ice cap into slush and ponds of water, has expanded inland, reaching elevations more than a mile high in some places, said Dr. Konrad Steffen, a glaciologist at the University of Colorado.

Recent measurements by NASA scientists show that such melting can have outsize effects on the ice sheet. Meltwater formed on the surface each summer percolates thousands of feet down through fissures, allowing the ice to slide more easily over the bedrock below and accelerating its slow march to the sea.

Some jutting tongues of floating ice, where riverlike glaciers protrude into the sea, are rapidly thinning. Measurements this year by Dr. Steffen and others on the Petermann Glacier in northern Greenland show that more than 150 feet of thickness melted away under that tongue in the last year. Such melting can speed the seaward movement of ice in the same way that removing a doorstop lets a door swing freely.

As Dr. Steffen settled in with three colleagues for weeks of grueling research at this half-buried wind-tattered camp 4,000 feet up the flanks of the ice cap, he described how other Greenland glaciers were speeding their discharge of icebergs into the sea.

''If other ice streams start to react in a similar way,'' he said, ''then we will actually produce much more fresh water.''

This influx of fresh water could block North Atlantic currents that help moderate the weather of the Northern Hemisphere. ''If that feedback kicks in,'' he said, ''then the average person will worry.''

Some oceanographers say global warming may already be pushing the North Atlantic toward instability. In less than 50 years, waters deep in the North Atlantic and Arctic have become significantly fresher, matched by growing saltiness in the tropical Atlantic. Worldwide, seas have absorbed enormous amounts of heat from the warming atmosphere. A big outflow of water from Greenland could take the system to a tipping point, some say.

In past millenniums when such oceanic breakdowns occurred, the climate across much of the Northern Hemisphere jumped to a starkly different state, with deep chills and abrupt shifts in patterns of precipitation and drought from Europe to Venezuela. Some changes persisted for centuries.

But whether something similar is likely to result from the new melting in Greenland is far from clear. The forces that caused abrupt climate change in the past, like monumental floods released from collapsing ice-age glaciers, are different from the much slower ones being measured today.

Gaps in understanding are enormous. Scientists have been unable to devise computer simulations that consistently replicate past jolts to the climate, leaving intellectual heartburn about the future.

''The models are not nearly as sensitive as the real world,'' Dr. Richard B. Alley, an expert at Penn State on Greenland's climate history, said. ''That's the kind of thing that makes you nervous.''

To solve the riddles, Greenland is being measured and monitored as never before, by satellites, by aircraft and by dozens of down-swaddled scientists who are braving 30-below-zero temperatures and deadly snow-cloaked crevasses that corrugate the slumping edges of the ice cap. The oceans around the ice sheet, which is four times the size of California, are peppered with instrument-laden buoys and plumbed with deep-diving robotic gliders.

The records obtained so far are so limited that no one can yet say whether the observed changes are a dangerous trend or a result of fluctuations in the naturally turbulent seas and atmosphere. Still, many experts who work on abrupt climate changes display a common concern about the relentless buildup of heat-trapping greenhouse gases in the atmosphere. While the human contribution to change here remains unknown, many scientists say the potential risks are all too evident.

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''It's as if we're running a gigantic uncontrolled experiment on the climate system and we know we have the capability of making large impacts,'' said Robert Hawley, a glaciologist from the University of Washington stationed at the Summit Camp, a permanent dome-topped installation at the highest point of the ice sheet. ''Should we continue just to experiment or should we try to figure this out first?''

The greatest concern is that the changes are part of an Arctic-wide pattern that includes a pronounced thawing of the Alaskan tundra and a retreat of the sea ice around the North Pole, at least in summer. Greenland's temperatures had for years bucked the Arctic warming trend. But they now seem to be increasing, according to new satellite data analyzed by Dr. Josefino C. Comiso at NASA's Goddard Space Flight Center.

The intimate relationship between Greenland and its surrounding seas was evident to 19 scientists, aides and other passengers on a recent flight from upstate New York aboard an Air National Guard LC-130 cargo plane. As the plane droned across the Labrador Sea separating North America from the white-topped island, the water below was laced with cordons of icebergs heading south from the ever-eroding edge of the ice cap, following the course taken in 1912 by the berg that sank the Titanic.

Just as Greenland contributes meltwater and melting icebergs to the sea, the northward-flowing surface currents of the Atlantic transport heat from the tropics to the Arctic and adjacent regions. As this heat is shed to the air, it renders Greenland's coasts relatively mild and helps make northern Europe warm enough that roses can be grown at latitudes that elsewhere support polar bears. As the water chills, it grows denser and sinks. The downward flow is the engine that powers a world-spanning oceanic ''conveyor belt'' through which seawater is eventually mixed and recycled.

Several times in the past, even early in the current 12,000-year warm spell in which modern civilizations arose, that conveyor belt has shut down. When it stopped, it stopped quickly, as if a circuit breaker had been tripped. In each case, according to recent research on layers of sea-bottom fossils and sediments, the trigger was a big inflow of fresh water into those vital spots where the surface water sinks.

Waters in the region are not only freshening, but a buildup of fresher water is diminishing the bank of saltier, denser water that sustains the deep southward-flowing circulation in the Atlantic, said Dr. Ruth Curry, an oceanographer at the Woods Hole Oceanographic Institution at Cape Cod, Mass.

In December, Dr. Curry, with British and Canadian researchers, reported the 40-year trend toward a freshening of the North Atlantic and growing saltiness in the tropics. She said that the Atlantic was being pushed toward ''a precipice'' that could be reached in three or four decades and that a Greenland meltdown could push it over the edge.

Many scientists disagree, saying even a significant melting is not likely to produce big shifts, at least in this century.

Dr. Peter Wadhams, an ocean physicist at Cambridge University, said a host of different mechanisms would keep the conveyor moving, even if the freshening persists. One is the formation of sea ice, which releases the salt in seawater as it forms, making underlying waters saltier and, thus, denser. Plumes of this briny water ''sink like syrup falling off a spoon,'' Dr. Wadhams said.

One possibility, some scientists say, is that the system could be self-regulating, with the shutdown of warm currents, in turn, cooling Greenland and stopping the melting. Another wild card, some computer models show, is that further warming will increase snowfall and add as much frozen water to Greenland as is lost through melting or the splitting off of icebergs. Other models suggest that the ice shield could shrivel entirely in 1,000 years, a phenomenon that scientists theorize has not occurred for 440,000 years.

On Greenland, efforts are under way to resolve the big uncertainties. Dr. Joseph R. McConnell, a snow hydrologist from the Desert Research Institute in Reno, Nev., is examining whether forces other than global warming could be responsible for changes here. Under a NASA grant, he recently completed a three-week 250-mile snowmobile crossing of the ice sheet, extracting four shaftlike ice cores en route. The cores have hundreds of layers, like tree rings, made by each year's snowfall. They will be analyzed for shifts in dust types or subtle chemical markers that could show whether winds were blowing from Europe or North America in particular years. The study may show if recent changes in snowfall and melting match natural cycles in North Atlantic weather.

After flying down from the summit to a coastal airfield following his snowmobile trek in May, Dr. McConnell took a break and hiked over undulating tundra to the clifflike edge of the ice. The spring warmth was eating away at the vaulting blue-white ramparts streaked with gray layers deposited by volcanoes and dust storms eons years ago. A torrent of silty meltwater poured along the bluff of ice, heading toward the Labrador Sea, 115 miles west.

''If Greenland melted,'' he said, ''it'd raise sea levels by 20 feet. There goes Florida. There goes most of the Mississippi embayment. There go the islands in the South Pacific. Bangladesh is obliterated. Manhattan would have to put up dikes.''

Although much work is needed to solve the puzzle, he added, the hints of big changes are building. ''There's definitely a lot of melting going on,'' Dr. McConnell said, flinching as a crack echoed from the warming white cliff and a giant ice slab shifted ever so slightly.

Correction: June 16, 2004, Wednesday A chart in Science Times on June 8 with an article about widespread climate changes that may be resulting from warmer weather in Greenland misstated the density of water in the North Atlantic, which is growing less salty as glaciers melt. The density ranges from 1,027 kilograms per cubic meter for the freshest water to more than 1,028 for the saltiest -- not from 27 kilograms to more than 28.